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(11) | EP 0 399 407 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Impermeable coating for refractory material, coated piece of this material and coating process |
| (57) Coating impermeable to air at the operating temperatures applied on a piece of refractory
material for the continuous casting of steel so as to reduce the aspiration of air
due to the permeability of the refractory. This coating is preferably comprised of
viscous phases at the operating temperature of the piece of refractory material.
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[0001] The pieces of refractory material used for the continuous casting of molten metals, notably steel, are used to protect the stream of molten metal flowing from the ladle toward the distributor from direct contact with the air. These refractory pieces are subjected to a substantial thermal shock at the beginning of the casting, as well as erosion and chemical corrosion by the molten steel and the slag. They are then manufactured into materials capable of resisting this aggressive environment. The materials most used are fused silica and graphitized alumina or a composite material of graphitized alumina and graphitized zirconia. The first material is used basically because of its exceptional resistance to thermal shock, but has limitations with regards to its erosion resistance for certain types of steel. The second material is much used because it permits longer casting times with aggressive steels.
[0002] Although the use of pieces of refractory materials, notably stream protection tubes, has considerably improved the quality of steels by avoiding direct contact of the molten metal stream with the air, a problem of air aspiration through the permeability of the material persists due to the aspiration effect that occurs in the runner under the effect of a rapid flow of the stream of liquid metal. This aspiration of air has the effect of oxidizing the molten metal and leading to the formation of alumina deposits in the runner. These deposits can range up to a complete plugging of the runner and an interruption of the casting. In addition, this air aspiration results in a degradation of the steel quality through the formation of inclusions of oxide and gas bubbles entrained in the stream. These bubbles induce needle-like holes in the billets, blooms or slabs. Another negative effect of air aspiration is to increase the nitrogen takeup in the steel between the ladle and the mold.
[0003] Attempts have been made to remedy the above shortcomings by reducing the permeability of refractory materials by the addition of groups of components that produce liquid phases at the operating temperature in order to stop up the pores of the refractory material. Nevertheless, this process has the disadvantage that the liquid phases diminish the hot properties of the refractory material, which results in a higher wear level of the refractory material by the steel and the slag. In view of the fact that attempts are currently being made to increase the service life of the refractory material to permit longer casting sequences, this solution is not acceptable.
[0004] The precise purpose of the invention is a coating material designed to be applied to a piece of refractory material for the continuous casting of steel that remedies the problem of the negative effect of air aspiration on the refractory material on which it is applied and, in particular, without reducing its resistance to erosion.
[0005] To this end, the invention concerns a coating applied to a piece of refractory material for the continuous casting of steel, characterized in that it is impermeable to air so as to reduce air aspiration due to permeability of the refractory.
[0006] The coating material is preferably comprised of viscous phases at the operating temperature of the piece of refractory material.
[0007] According to a first preferred embodiment example, the refractory material of the invention comprises an aqueous suspension containing ca. 30-85 wt.% of a finely divided constituent chosen among fused silica grains, alumina powder, zirconia powder, mullite powder and alumina droplets, ca 0-10 wt.% of a ceramic charge chosen among the fibers of alumina silica, zirconia fibers, titanium dioxide fibers, fibers of chromium alumina, alumina droplets and zirconia droplets, ca. 15-30 wt.% water, 0-7 wt.% binder chosen among sodium hexametaphosphate, sodium silicate and acrylic resins, and 0-40 wt.% of a generating frit of glass.
[0008] A second example of preferred embodiment of a coating according to the invention essentially presents the following composition by wt : 50-80% SiO₂, 5-15% Al₂0₃, 5-20% B₂0₃, 1-3% K₂0, 0-15% Na₂0, 0.5-2% Fe₂0₃, 0-3% Co₂0₃/Mo0₃, 1-5% C and 0-1.5% others.
[0009] The coating of the invention can be comprised of one or more layers of different materials. For example, it can be comprised of a layer according to the first preferential embodiment example, and one or more layers according to the second preferential embodiment. It can also be comprised of several layers of the same material, successively dried before the addition of the following layer. Finally, it is also possible to incorporate a cement layer in the coating.
[0010] According to one actualization variant, the coating of the invention has a layer of conventional anti-oxidation claying on which at least another layer of impermeable material is applied. Finally, the coating can be comprised of a thick layer of dried claying and then covered with a cement.
[0011] The invention also concerns a piece of refractory material having an impermeable coating according to the invention. It also concerns a coating process for a piece of refractory material by means of the said coating. According to this process, a coating is prepared according to the invention and a piece of refractory material is covered with it by means of a brush, by immersion, pressing or by electrodeposition.
EXAMPLE 1
[0012] Two curves are plotted in Fig. 1 for comparison of the permeability of a piece of refractory material of graphitized alumina covered only with a conventional anti-oxidation layer (curve A) and the permeability of a piece of refractory material coated with an impermeable coating according to the invention (curve B). In this actualization example, a coating was used on the basis of fused silica as described in the French Patent no 8709023, filed on June 26, 1987 and concerning an insulating coating for a refractory body. This coating was applied by immersion. The refractory piece was first coated with a first layer of conventional anti-oxidation claying. A suspension comprised essentially (by wt.) of 30-85 % vitreous silica grains, 0-10% ceramic fibers, 0-7% binder, 0-4% frit and 15-30% water was then applied. After drying, the thickness of the coating layer was between 1 and 2 mm and had the following compositions :
96% SiO₂
4% Al₂0₃.
[0014] As can be seen in FIg. 1, due to the use of the coating material according to the invention, the permeability was reduced under the test conditions form ca. 350 ml/min in the case of the piece of refractory material of graphitized alumina having only one conventional anti-oxidation layer to ca. 100 ml/min at 1000°C to be cancelled above 1140°C and for higher temperatures, for a refractory material of graphitized alumina having the same composition.
[0015] According to another actualization example (not shown), the same coating material was applied by electrodeposition. With this process the permeability was reduced to ca. 35-40 ml/min above 1200°C.
EXAMPLE 2
[0016] Figure 2 plots two curves that give the flow rate in ml/min of argon through pieces of refractory material as a function of the temperature. The first curve, designated by the letter C, is relative to a piece of refractory material of graphitized alumina coated only with a conventional anti-oxidation layer. The second curve, designated by the letter D, refers to a piece of refractory material of the same composition covered with a composite impermeable coating according to the present invention. With regard to curve C, the experimental conditions are the same as in the first actualization example (curve A), the flow rate is the same, i.e., approximatively 350 ml/min above 1200°C. It is found that the composite coating (curve D) permits of a quite substantial diminution in this permeability. It passes from 350 ml/min to 55 ml/min at 1100°C and to 35 ml/min at 1210°C and above.
[0017] This layer of composite coating was comprised of a triple layer of claying, applied by immersion on the refractory before baking and a layer of cement applied with a brush on the claying layer after baking. The composition of the claying layer was a follows :
SiO₂ : 56,4 %
B₂0₃ : 12,3 %
AL₂0₃ : 9,7 %
K₂0 : 2,7 %
Na₂0 : 10,1 %
Fe₂0₃ : 0,8 %
Co₂0₃/MoO₃ : 2 %
C : 4,9 %
[0018] The cement layer had the following composition :
Al₂0₃ : 45 %
SiO₂ : 48,9 %
Fe₂0₃ : 1,2 %
TiO₂ : 1,9 %
Na₂0/K₂0 : 2,5 %
[0019] The thickness of the claying layer was ca. 0,5 mm and the thickness of the cement layer, ca. 2 mm. A coating having only three layers of claying without the addition of a supplementary layer of cement was also tested. This coating exhibited a lesser production of permeability that was reduced under the same experimental conditions only to 70 ml/min above 1140°C.
[0020] In general, the reduction in permeability can be obtained by virtue of a coating that produces viscous phases in the range of operating temperature. The best results are obtained when the viscosity of the coating is just sufficiently low to permit a closing of its porosity, as well as the porosity of the refractory material. In effect, if the viscosity of the coating material continues to decrease, this coating flows along the walls of the piece. Its thickness diminishes to the point that the layer becomes permeable. A good interval of viscosity is considered to lie between 3000 and 5000 pise for such a coating. The application of a surface layer of refractory cement as mentioned in Example 2 can permit the use of more liquid underlying materials. In effect, the flow of these more liquid materials is rendered difficult by the surface cement.
1. Coating applied on a piece of refractory material for the continuous casting of
steel, characterized in that it is impermeable to air at the operating temperature
of the piece of refractory material so as to reduce the aspiration of air due to the
permeability of the refractory material.
2. Coating according to claim 1, characterized in that it is comprised of viscous
phases at the operating temperature of the piece of refractory material.
3. Coating according to claims 1 or 2, characterized in that it presents essentially
the following composition, by wt :
SiO₂ : 50 to 80 %
Al₂0₃ : 5 to 15 %
B₂0₃ : 5 to 20 %
K₂0 : 1 to 3 %
Na₂0 : 0 to 15 %
Fe₂0₃ : 0,5 % to 2 %
Co₂0₃/Mo0₃ : 0 to 3 %
C : 1 to 5 %
Others : 0 to 1.5 %
SiO₂ : 50 to 80 %
Al₂0₃ : 5 to 15 %
B₂0₃ : 5 to 20 %
K₂0 : 1 to 3 %
Na₂0 : 0 to 15 %
Fe₂0₃ : 0,5 % to 2 %
Co₂0₃/Mo0₃ : 0 to 3 %
C : 1 to 5 %
Others : 0 to 1.5 %
4. Coating according to claim 1 or 2, characterized in that it is comprised of an
aqueous suspension containing ca. 30-85 wt.% of a finely divided constituent and chosen
among the grains of fused silica, alumina powder, zirconia powder, mullite powder,
and alumina droplets, 0 to ca. 10 wt.% of a ceramic charge chosen among the alumina-silica
fibers, zirconia fibers, titanium dioxide fibers, chromium-alumina fibers, alumina
droplets and zirconia droplets, ca. 15-30 wt.% of a generating frit of glass.
5. Coating according to any of claims 1-4, characterized in that it is comprised of
several layers of different materials.
6. Coating according to any one of claims 1-5, characterized in that it is comprised
of a convential anti-oxidation claying layer on which at least one layer of an impermeable
material is applied.
7. Coating according to one of claims 5 or 6, characterized in that it is comprised
of a thick layer of dried claying and then covered with a layer of cement.
8. Piece of refractory material, characterized in that it has a coating according
to any of claims 1-7.
9. Process for coating a piece of refractory material, characterized in that a coating
is prepared according to any of claims 1-7, and that the piece of refractory material
is covered with this coating.
10. Coating process according to claim 9, characterized in that the coating is applied
by brushing, immersion, pressing or electrodeposition.